The present embodiments relate to a method and a control sequence determination facility for determining a magnetic resonance system activation sequence.
In a magnetic resonance system, the body to be examined may be exposed to a relatively high basic magnetic field known as the B0 field of 3 or 7 Tesla, for example, with the aid of a basic field magnet system. A gradient system is also used to apply a magnetic field gradient. High-frequency excitation signals (HF signals) are then emitted by way of a high-frequency transmit system using suitable antenna facilities. The aim is to tip the nuclear spin of certain atoms that have been excited in a resonant manner by the high-frequency field with spatial resolution through a defined flip angle in relation to the magnetic field lines of the basic magnetic field. The high-frequency magnetic field emitted in the form of individual pulses or pulse trains is also referred to as the B1 field. This magnetic resonance excitation (MR excitation) by magnetic high-frequency pulses and the resulting flip angle distribution are also referred to below as nuclear magnetization or “magnetization”. During relaxation of the nuclear spin, high-frequency signals (e.g., magnetic resonance signals) are emitted. The magnetic resonance signals are received using suitable receive antennas and are further processed. Raw data acquired in this manner may be used to reconstruct the desired image data. Emission of the high-frequency signals for nuclear spin magnetization may take place using a whole body coil or body coil. A typical structure of a whole body coil is a birdcage antenna consisting of a number of transmit rods disposed around a patient chamber of the tomography system, in which a patient is present during the examination. The transmit rods may run parallel to the longitudinal axis. End faces of the antenna rods are connected, respectively, in a capacitive manner in a ring. Local coils, however, may be used in proximity to the body to emit MR excitation signals. The magnetic resonance signals may be received using the local coils. Alternatively or additionally, the magnetic resonance signals may be received using the body coil.
Whole body antennas may be operated in a “homogeneous mode” (e.g., a “CP mode”). A single temporal HF signal with a defined fixed phase and amplitude ratio is emitted to all the components of the transmit antenna (e.g., all the transmit rods of a birdcage antenna). Individual HF signals may be assigned to the individual transmit channels (e.g., the individual rods of a birdcage antenna). A multichannel pulse that includes a number of individual high-frequency pulses is emitted in a parallel manner by way of the different independent high-frequency transmit channels. Such a multichannel pulse (e.g., a “pTX-pulse” due to the parallel emission of the individual pulses) may be used, for example, as an excitation, refocusing and/or inversion pulse. An antenna system having a number of independently activatable antenna components or transmit channels may also be referred to as a “transmit array,” regardless of whether the antenna system is a whole body antenna or an antenna arrangement disposed in proximity to the body.
Such pTX pulses or pulse trains made up of such may be determined beforehand for a certain planned measurement. To plan the HF pulse sequence, the user predefines an MR excitation quality (e.g., in the form of a target magnetization). The user may, for example, predefine a desired flip angle distribution with spatial resolution. The predefined desired flip angle distribution is used within a target function as a setpoint value. The appropriate HF pulse sequence for the individual channels is then calculated in an optimization program (e.g., a “target function optimizer”), so that predefined MR excitation quality is achieved. A method for designing such multichannel pulses in parallel excitation methods is described, for example, in W. Grishom et al.: “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation”, Mag. Res. Med. 56, 620-629, 2006.
For a certain measurement, the different multichannel pulses, the gradient pulses associated with the respective activation sequence and further control defaults are defined in a measurement protocol. The measurement protocol is produced beforehand and may be called up for a certain measurement from a memory, for example, and optionally changed by the operator in situ. During the measurement, the magnetic resonance system is controlled fully automatically on the basis of this measurement protocol, with the control facility of the magnetic resonance system reading the commands out of the measurement protocol and processing the commands.
Every MR excitation results in high-frequency exposure of the patient, which must be limited according to certain rules. Too great a high-frequency exposure may harm the patient. HF exposure refers not only to the HF energy input but also to physiological exposure induced by the HF irradiation. A typical measure of high-frequency exposure is the specific absorption rate (SAR) value that indicates in watts/kg the biological exposure acting on the patient due to a certain high-frequency pulse output. Another measure of high-frequency exposure if the specific energy dose (SED) value. The two values are readily converted to one another.
For the emission of homogeneous fields, a global high-frequency exposure may be monitored. For example a standardized limit of 4 watts/kg at the “first level,” according to the IEC standard, applies for the global SAR exposure of a patient. Since during the emission of pTX pulses in the patient the previously homogeneous excitation may be replaced by any form of excitation, “hotspots,” where the high-frequency exposure may be many times the values known previously from homogeneous excitation, may form. With pTX transmit methods, both global and local high-frequency exposure is to be taken into account.
The high-frequency exposure of the patient is monitored continuously on the magnetic resonance system during the examination using suitable safety devices, and a measurement is changed or terminated if the monitored HF exposure value is above the specified standards. The expected HF exposure is to be estimated as accurately as possible during prior planning, and to be taken into account when determining the magnetic resonance system activation sequence to avoid exceeding the limit values during measurement. The interruption of measurement necessitates a new measurement.
The maximum permitted high-frequency exposure is a significantly limiting factor in MR imaging (e.g., at higher field strengths). For example, the pulse energy and therefore also the high-frequency exposure caused by an HF pulse may be reduced by lengthening the relevant high-frequency pulse. However, lengthening an HF pulse narrows the band of its frequency spectrum. The option of shortening the pulses is therefore limited by the spatial inhomogeneity of the B0 field, since the MR excitation frequency is a function of the B0 field strength. Conventionally, therefore, within the scope of possible output limitations, the HF pulses are as a maximum, made so short that the B0 field inhomogeneities may be ignored. There has been no provision to date for pulse shortening beyond this to reduce the high-frequency exposure.